Lithium alloy/iron sulfide bipolar batteries, currently under development, have positive and negative electrode materials confined relative to structural positive and negative current collectors, which are electrically insulated from one another by separators. Typically, the negative electrode material is a lithium alloy (generally LiAl or LiSi), the positive electrode material is an iron sulfide (FeS or FeS.sub.2), and the separators are formed of a fibrous boron nitride (BN) or a pressed powder magnesium oxide (MgO) or aluminum nitride (AIN). An electrolyte such as a lithium chloride, lithium bromide and potassium bromide mixture (LiCl-LiBr-KBr), is present in the electrode materials and the separators. The positive and negative current collectors are commonly formed of electrically conductive sheets that also confine the electrode materials.
Full size batteries of this type are comprised of many cells grouped together in an end-to-end or face-to-face arrangement in a common battery housing and electrically connected in series to produce higher effective voltage output. A thin cell version is capable of very high current density. The battery is designed to operate at temperatures in the range of 375.degree.-500.degree. C. The electrode materials and electrolyte are most corrosive at these temperatures so that the current collectors must be of corrosive resistant yet electrically conductive material. Moreover, the battery is intended to have an operating life in excess of 1000 "deep discharge" cycles, where each "deep discharge" cycle means discharging the fully charged battery down to approximately only a 5% charge level before recharging it again. During this deep discharge cycling, the positive and negative electrode materials undergo volumetric changes at different rates. This can shift the electrode materials relative to one another within the battery cell or can even deform the separators.
Another major problem in existing bipolar battery designs and particularly those involving electrolytes normally fluid at cell operating temperatures (i.e., 375.degree.-500.degree. C.) has been electrolyte leakage past the wetted separator between adjacent positive and negative electrodes. The electrolyte is consumed by electrolytic decomposition and could produce metallic deposits sufficient to cause battery failure by shorting out the adjacent collectors or to the external battery housing. Compression of the stacked and sandwiched plate-like cell components within the battery case confinement is used now as the primary means in many bipolar batteries to maintain the separator sealed at its edge. Past approaches to hermetic sealing of the bipolar battery have not been effective. The cells were stacked first and then seal fabrication was attempted. Usually the cells contain electrolyte prior to the formation of seals adjacent the electrolyte. Since the sealing often occurs at elevated temperatures, electrolyte leakage limits the effectiveness of the seals. Seal fabrication conditions are also severely restricted due to the presence of electrode materials and electrolyte.
Accordingly, one object of this invention is to provide a design for a bipolar battery that seals the positive and negative electrode materials within a leak-proof current-collecting containment separated from another except across a separator module interposed therebetween.
A second object of the invention is to provide a cell enclosure design in which any seals adjacent the electrolyte cavity or chamber are formed prior to the addition of electrolyte.
A more detailed object of this invention is to provide for the stacked arrangement of many of such cells and further to mechanically and electrically connect them together, as part of the same containment design, with the positive and negative current collecting containments of adjacent cells sharing a common or bipolar current collector to provide also a series electrical connection of the cells.
A specific object of this invention is to provide an improved seal arrangement at the edge of each cell particularly in a high power lithium metal/iron sulfide battery.
Another object of this invention is to provide a seal arrangement for the anode and cathode electrodes by utilizing peripherial edges of the containment sheets of the separator module that project radially beyond the annular spacer and that can be folded over the edges of the corresponding anode and cathode electrode material and sealed to a current collector sandwiching the opposite side of the respective electrode material.
An additional object of this invention is to provide a method of forming the separator module without electrolyte or electrode materials present.